1. Field of the Invention
The present invention relates to a method and apparatus for processing a telephone Customer Premises Equipment Alerting Signal (CAS) and, more particularly, to a method and apparatus for detecting a CAS in the presence of a voice signal, verifying that the detected CAS is a valid CAS, and responding to the CAS with an appropriate acknowledge signal.
2. Description of the Related Art
Many telephone companies provide a Caller ID service wherein a person (hereinafter referred to as the called party, the subscriber, the local party, and the like) is provided with information concerning a calling party while the telephone of the called party is still ringing. For example, the telephone number and, in some cases, also the identity, e.g., name, of the calling party can be displayed on a device adjacent to the telephone before the called party answers the telephone.
To provide Caller ID service, the telephone company transmits the calling party information over a telephone line to the called party, typically between ring signals. In the U.S., the information is usually transmitted between the first and second ring signals. The information is transmitted in the voice band in the form of a Frequency Shift Keying (FSK) signal at 1200 baud. The calling party information is transmitted in an Incoming Calling Line Identification (ICLID) signal, which includes a channel seizure signal, a mark signal, and a data signal.
A unit of Customer Premises Equipment (CPE; located adjacent to or integral with the telephone) called a Caller ID device detects, decodes, and displays the ICLID signal prior to the start of the second ring signal. The called party can use the displayed information to decide whether or not to answer the call. In addition, the Caller ID device can store in a memory the called party information along with the date and time of the call for later review by the user.
Many telephone companies also provide a service called Call Waiting. Call Waiting is a service in which an audible alerting signal is provided to alert a called party, who is engaged in a telephone conversation with a first party, that a second party is attempting to place a call to the called party. The audible alerting signal is referred to as a Subscriber Alerting Signal (SAS) and typically has a frequency of 440 Hz. Upon hearing the SAS, the called party can choose to put the first party on hold and answer the new incoming call by executing a switch-hook flash. A telephone company Central Office recognizes the switch-hook flash, puts the first party on hold, and connects the second party to the called party. The called party can switch back and forth between the first and second parties by executing one or more switch-hook flashes.
Conventionally, Caller ID and Call Waiting services have been independent; Caller ID is provided when a telephone is on-hook, and Call Waiting is provided when a telephone is off-hook. However, many telephone companies will soon offer a new service which combines Caller ID with Call Waiting. This new service allows a called party who receives an SAS while engaged in a conversation with a first party to receive information regarding the second party who is trying to reach the called party. In this manner, upon hearing an SAS indicating another incoming call, the called party can observe the number, and in some cases the identity, of the second party. This information can assist the called party in determining whether to put the first party on hold and answer the second party's call.
The new service is referred to as Calling Identity Delivery on Call Waiting (CIDCW). A CIDCW protocol is outlined in Bellcore Special Report SR-TSV-002476 (Issue 1, December 1992), Bellcore Technical Reference TR-NWT-000030 (Issue 2, October 1992), and Bellcore Special Report SR-NWT-003004 (Issue 1, February 1994) (collectively referred to as "the Bellcore specification"). This specification outlines a signaling protocol that allows Caller ID information related to a new call to be transmitted to a customer while the customer is on line with an established call. The transmitted information is similar to an ICLID signal, although it does not need a channel seizure signal, and can be processed in a similar way. The Bellcore specification also provides CPE compatibility considerations for CIDCW. As discussed below, CPE devices for use with CIDCW must handle several complex problems that do not exist with conventional Caller ID.
Conventional Caller ID information, i.e., an ICLID signal, is easy to detect in the U.S. since it is typically transmitted after a first ring signal. The ring signal is easy to detect and can be relied upon to initiate a process for receiving the Caller ID information. Further, the Caller ID information is relatively easy to detect since it is transmitted in an on-hook state, i.e., prior to establishing a telephone line connection. There is relatively little noise on the line in the on-hook state since there is no voice signal.
In contrast, the calling party information provided for CIDCW must be sent whenever a second party attempts to place a call, rather than at a predetermined time. This means a CPE device at a called party's location must be alerted when calling party information is to be sent. To address this timing problem, the Bellcore specification specifies that a CPE Alerting Signal (CAS) be sent to the called party by the Central Office just after sending the 440 Hz SAS. The CAS is a dual tone multi-frequency (DTMF) signal consisting of a 2130 Hz tone and a 2750 Hz tone and has a duration of 80 to 85 ms. When the CAS is detected by a CPE device at the called party's location, the local telephone is muted while a DTMF "D" acknowledge signal, consisting of a 941 Hz tone and a 1633 Hz tone, is sent back to the Central Office to initiate transmission of the calling party information. The local telephone remains muted while the calling party information is transmitted by the Central Office in response to the acknowledge signal.
The muting of the local telephone, which occurs after a CAS has been detected, prevents interference with the acknowledge signal and the calling party information signal. In addition, since the calling party information signal produces an unpleasant, raucous noise in the telephone, the muting ensures that the called party does not have to listen to that noise.
One problem posed by the Bellcore specification is that the CAS must be detected in the presence of voice signals during an established call. This is a much more difficult task than detecting conventional Caller ID information in a relatively quiet, on-hook state. The Central Office can mute the voice signal of the distant party currently on-line with the local party before transmitting the CAS. However, the local voice signal of the called party, i.e., a near-end speech signal, will often be present during transmission of the CAS. Typically, the signal-to-noise ratio (SNR) for a CAS received at the called party's location will be a negative value, and may be quite low. In the worst case, the CAS may be at a level of -32 dB when received. Since voice signals may be present at -7 dB, a worst case SNR for the CAS may be -25 dB. An SNR of -20 dB will not be uncommon. Further, there may be a power differential between the two CAS tones, which may be as high as 6 dBm.
It is very difficult for conventional DTMF detectors to effectively detect a DTMF signal such as the CAS in the presence of voice signals. Such detectors separate an input signal into an upper and a lower band by using a low pass filter and a high pass filter. The filter outputs are passed to an appropriate detector. The input filters can be analog, digital, switched capacitor, and the like. The detectors can be a peak-to-peak type, phase-locked-loop (PLL) type, and the like. However, such a conventional arrangement cannot reliably detect a brief CAS having a low SNR, i.e., a CAS that is attenuated and interfered with by a voice signal.
Another significant problem is that a voice signal can easily mimic the CAS and cause a false detection. This presents a problem since the local telephone is muted each time a CAS is detected and an acknowledge signal is sent. If many false detections occur, a telephone conversation may repeatedly be interrupted by unnecessary muting, producing a very annoying distraction for the conversing parties.
The majority of false detections are caused by a local voice signal, i.e., near end speech. This is because the distant party's voice signal is attenuated by the communication network, which makes it less likely to produce a false detection. Therefore, the CIDCW service would be vastly improved by eliminating false detections caused by a local signal.